A carbon dioxide absorbent is typically produced by mixing hydrated lime, Ca(OH)2, with water and optionally a small amount of sodium or potassium hydroxide to form a paste, which is then extruded or molded into particles, in granular or pellet form, approximately 2-3 mm in diameter and 2-5 mm in length. So-called soda lime absorbents are typically used in hospital operating rooms for inhalation anesthesiology, in recovery room re-breathing devices, and in underwater breather systems and devices. They are loaded in loose particulate form or contained within convenient disposable cartridges.
To indicate the progressive exhaustion of the absorbent, a color indicator dye which is sensitive to pH, such as diethyl violet (DEV), is added during manufacture. The dye in this case changes from a colorless state to the color purple as absorption proceeds. The state of substantial exhaustion of the carbon dioxide absorption capacity is indicated by a deep purple color. However, when the absorbent is allowed to sit idle for several hours or more after usage, the purple color can slowly fade and revert to a near colorless state. This renders it difficult for medical personnel to ascertain the absorption capacity remaining, although the purple color will eventually return when the absorbent is again exposed to carbon dioxide.
Thus, it is an objective of the present invention to prevent or minimize the reversion to colorlessness of the indicator dye.
Another objective of the present invention to prevent or minimize degradation of certain anesthetic agents. Carbon dioxide absorbents such as soda lime can cause certain anesthetic agents (e.g., sevoflurane) to degrade slightly by extracting an HF molecule to form an undesired olefin byproduct referred to as “Compound A” with the formula CF2═C(CF3)OCH2F. Soda lime that contains extremely low levels of moisture can also cause other volatile anesthetic agents, such as desflurane, enflurane, and isoflurane, to degrade and form carbon monoxide.
It is also an objective of the present invention to provide a calcium hydroxide-containing absorbent that minimizes the degradation of certain volatile anesthetic agents to either Compound A (an undesired byproduct) or carbon monoxide.
U.S. Pat. No. 4,407,723 of MacGregor et al. disclosed a method for making carbon dioxide absorbents. Pure calcium hydroxide and water were mixed into a paste, extruded through a grate (e.g., meat grinder), air-dried into hardened granules, and then sized through sieves to obtain uniform size. Subsequently, an aqueous solution containing sodium hydroxide, potassium hydroxide, calcium chloride, and water was sprayed and absorbed onto the granules. Thus, the method required an extra manufacturing step, and also did not guarantee that all surfaces of the particles were sufficiently treated.
Thus, it is another objective of the present invention to provide for convenience and efficiency in the manufacturing of absorbent particles.
In U.S. Pat. No. 6,228,150, Armstrong et al. disclosed a carbon dioxide absorbent that included calcium hydroxide and a “humectant.” The humectant was considered to be either “hygroscopic” (which meant that it absorbed atmospheric water) or “deliquescent” (which meant that it absorbed atmospheric water and dissolved in the water thus absorbed). Preferred by Armstrong et al. was calcium chloride as a humectant. Armstrong et al. also wanted their calcium hydroxide-based absorbents essentially free of sodium and potassium hydroxide, purportedly to avoid carbon monoxide and Compound A arising from degradation of anesthetic agents.
Thus, it is a further objective of the present invention to avoid substantial degradation of anesthetic agents, while also providing the option of employing sodium and/or potassium hydroxide in the absorbent composition to improve carbon dioxide absorption efficiency.
Thus, a novel absorbent composition and method of manufacture are needed to avoid certain disadvantages of the prior art as mentioned above.